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PangaeaNearly 100 years ago, a scientist named Alfred Wegener

(VAY guh nuhr) began an investigation. He wanted to know if Earth’s continents had always been in the same place, or if they had moved. Wegener proposed that all the continents were once part of a supercontinent called Pangaea (pan JEE uh). Over time, Pangaea broke apart, and the continents slowly drifted to their present locations. Wegener proposed the hypothesis of continental drift. The continental drift hypothesis suggested that continents are in constant motion on the surface of Earth.

Wegener looked at the coastlines of continents that are now separated by oceans. He saw similarities in their shapes. For instance, Africa and South America seemed to fit together like the pieces of a puzzle, as shown below.

What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.

Before Statement After

1. India has always been north of the equator.

2. All the continents once formed one supercontinent.

Key Concepts • What evidence supports

continental drift?• Why did scientists question

the continental drift hypothesis?

Continental

shelf

Identify the Main Ideas Highlight two or three phrases in each paragraph that summarize the information presented. After you have finished the lesson, review the highlighted text.

Visual Check1. Identify With a pen or pencil, trace the area where Africa and South America match up.

The Continental Drift Hypothesis

Plate Tectonics

Make a half-book. Use it to organize your notes on the continental drift hypothesis.

Evidence forthe ContinentalDrift Hypothesis

LESSON 1

CHAPTER 7

Reading Essentials Plate Tectonics 91

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Evidence That Continents MoveWegener knew that he needed evidence to support his

hypothesis of continental drift. The most obvious evidence was how the continents fit together like pieces of a puzzle. But other scientists were doubtful of his hypothesis. Wegener needed more evidence.

Fossil CluesThere are many animals and plants that live only on one

continent. For example, lions live in Africa but not in South America. Because oceans separate the continents, animals cannot travel from one continent to another by natural means. However, fossils of similar organisms have been discovered on several continents that are now separated by oceans.

Fossils of a plant called Glossopteris (glahs AHP tur us) have been discovered in rocks from South America, Africa, India, Antarctica, and Australia. Today these continents are far apart and separated by oceans. The plant’s seeds could not have traveled across the oceans.

The figure below shows how some of the continents were joined as part of Pangaea 250 million years ago. The lighter area on the map shows where Glossopteris fossils have been found. Notice that the plant once grew in parts of five continents—South America, Africa, India, Antarctica, and Australia. Because these plants grew in a swampy environment, this region, including Antarctica, was different from how it is today. Most of Antarctica is covered in ice sheets. No swampy environments are found there now.

2. Specify Where did Glossopteris probably grow?

REVIEW VOCABULARYfossil the naturally preserved remains, imprints, or traces of organisms that lived long ago

Visual Check 3. Consider Which of the continents would not support Glossopteris growth today?

Africa

India

Antarctica

Australia

South

America

92 Plate Tectonics Reading Essentials

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Climate CluesOther fossil evidence supported continental drift. Coal

beds are in Antarctica, a polar climate today. Yet coal formed from fossilized plants that lived long ago in warm, wet climates. This meant that Antarctica must have been warmer and wetter when these plants were alive. Is it possible that Antarctica was at one time closer to the equator? Did Antarctica move to a colder climate near the South Pole?

Another climate clue used by Wegener to support continental drift came from glaciers. When Wegener pieced Pangaea together, he proposed that South America, Africa, India, and Australia were located closer to the South Pole 250 million years ago. He suggested that a large ice sheet covered much of the continents, as shown below. When the ice sheet melted as Pangaea spread apart, it left rock and sediment behind. Wegener studied the similarities of these sediments.

Wegener also studied glacial grooves. Glacial grooves are deep scratches in rocks made as ice sheets move across the land. Wegener found glacial grooves on many different continents. By studying these grooves, he was able to determine the direction that the ice sheet moved across the joined continents.

Reading Check4. State Why did Wegener suggest that continents in the southern hemisphere had a colder climate long ago?

Visual Check5. Name the areas on Earth where there is evidence of ancient glaciers.

Past

Present

Ice mass

Australia

Australia

Asia

Asia

Africa

Africa

Europe

Europe

NorthAmerica

NorthAmerica

SouthAmerica

SouthAmerica

India

India

Antarctica

Antarctica

Equator

Equator

Rock CluesSome of the evidence used by Wegener to support his idea

of continental drift came from rock formations on different continents. The rock formations and mountain ranges seemed to have formed in the same way at the same time. Today geologists know that there were large-scale volcanic eruptions on the western coast of Africa and on the eastern coast of South America hundreds of millions of years ago. Geologists have studied rocks from these eruptions. They found that the volcanic rocks from both continents were identical in chemistry and age.

Reading Essentials Plate Tectonics 93

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Key Concept Check7. Specify How were similar rock types used to support the continental drift hypothesis?

Mountain Chains More evidence came from the rocks that make up two mountain chains in Europe and North America. Locate the caledonian mountain range and the Appalachian Mountains in the figure below on the left. The caledonian mountain range is in northern Europe, and the Appalachian Mountains are in eastern North America.

Visual Check6. Name Which mountain range is in Europe?

Key Concept Check8. Explain Why did scientists argue against Wegener’s continental drift hypothesis?

NorthAmerica

Greenland

Eurasia

AfricaSouthAmerica

CaledonianMountains

AppalachianMountainsNorth

America

BritishIsles

Scandinavia

Greenland

Eurasia

Africa

CaledonianMountains

AppalachianMountains

Rock Types Rocks in these two mountain chains are similar in age and structure. Both are also composed of the same rock types. If you could place North America and Europe next to each other, these mountain chains would meet. They would form one long, continuous mountain belt, shown in the figure above on the right.

What was missing? Wegener supported his continental drift hypothesis until

his death in 1930. Wegener’s ideas were not widely accepted until nearly 40 years later. Why were scientists skeptical of Wegener’s hypothesis?

Continental drift is a slow process. Wegener could not measure how fast the continents moved. Wegener also could not explain what forces caused the continents to move. The mantle under the continents and seafloor was made of solid rock. How could continents push their way through solid rock? Wegener needed more scientific evidence to prove his hypothesis.

The evidence for drifting continents was hidden on the seafloor. During Wegener’s lifetime, scientists did not have the tools to determine what happened beneath the oceans. Wegener also could not have known what the seafloor looked like. The evidence needed to prove continental drift was not discovered until long after Wegener’s death.

Mountain Ranges

94 Plate Tectonics Reading Essentials

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Mini Glossary

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.

ConnectED

1. Review the terms and their definitions in the Mini Glossary. Write two sentences that explain how Pangaea and continental drift are related.

2. Draw three types of evidence on the map that support Wegener’s hypothesis of continental drift. Highlight each drawing with a different-colored marker. Make a key next to the map that shows what each color means.

continental drift: a hypothesis suggesting that continents

are in constant motion on the surface of Earth

Pangaea (pan JEE uh): one supercontinent that all the

continents were once part of

3 . How did highlighting one or two phrases in each paragraph help you understand the hypothesis of continental drift?

Pacific Ocean

Active volcano

Earthquake epicenter

Plate boundary

Europe

Africa

Asia

Australia

NorthAmerica

SouthAmerica

Reading Essentials Plate Tectonics 95

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